Concentration in molarity?

Question

Calculate the concentration in molarity of a NaOH solution if 10 mL of the solution are needed to neutralize 21 mL of a .312 molar HCl solution?

Answer 1

Let NaOH solution be called NS. Let HCl solution be called HS.

21mLHS/10mLNS x 0.312molHCl/1000mLHS x 1molNaOH/1molHCl x 1000mLNS/1LNS = 0.655 moles NaOH per 1L NS solution

Answer 2

Moles HCl = 21 mL x 0.312 / 1000 = 0.00655
HCl + NaOH >> NaCl + H2O
the ratio between HCl and NaOH is 1 : 1
Moles HCl = moles NaOH = 0.00655
10 mL = 0.010 L
M = moles / L = 0.00655 / 0.010 = 0.655